An electronic control unit (ECU) for controlling a vehicle by CAN that is one of in-vehicle local area networks (LAN) is constructed to be selectively operable in a standby mode for low power consumption and a normal mode. Communication data are transmitted through a bus and compared with a threshold by a comparator.
Even in the standby mode, when the ECU receives data transmitted from another ECU or the like in the vehicle, the ECU is switched to the normal mode (wake-up operation) in response to the reception of such data. The comparator therefore needs be held operable.
The comparator necessarily consumes electricity if held operable even in the standby mode. The comparator in the standby mode is only required to output a trigger for switching the ECU to the normal mode when the data is received. The comparator thus need not have a quick response characteristic.
It is proposed to provide for the standby mode another comparator, which is a low power consumption and slow response characteristic, in addition to the comparator for the normal mode. The two comparators are switched over to be selectively operable in such a manner that the power consumption in the standby mode is reduced.
FIG. 6 schematically shows a proposed arrangement of two comparators 1 and 2. The comparators 1 and 2 are for a normal operation and a standby operation, respectively. Each comparator 1, 2 receives data at a non-inverting input terminal (IN) and compares it with a threshold TH1, TH2 thereby to output either a high or low level signal. Each output terminal B, C is connected to an input port of a microcomputer 4 through a multiplexer (MPX) 3. The microcomputer 4 is constructed to receive data through the comparator 1 in the normal mode for communication with other devices through an in-vehicle LAN. The thresholds TH1 and TH2 may be set to the same levels or different levels. The threshold TH2 is preferably set to be lower than the threshold TH1, because the comparator 2 is a slow response type.
The microcomputer 4 outputs a mode switching signal A to select either one of the comparators 1 and 2 as an operating comparator and select either one of the output signals by the multiplexer 3 as an output D of the multiplexer 3. For instance, the microcomputer 4 sets the mode switching signal A to a low level for the standby mode operation as shown by (A) of FIG. 7, so that only the comparator 2 is held operable as shown by (C) of FIG. 7 and the multiplexer 3 selects the output terminal C, that is, the output signal of the comparator 2 as shown by (D) of FIG. 7. The microcomputer 4 sets the mode switching signal A to a high level for the normal mode operation as shown by (A) of FIG. 7, so that only the comparator 1 is held operable as shown in (B) of FIG. 7 and the multiplexer 3 selects the output terminal B, that is, the output signal of the comparator 1 as shown by (D) of FIG. 7.
In the above proposed arrangement, when the comparator 2 receives data in the standby mode, it outputs the received data to the microcomputer 4. The microcomputer 4 wakes up from the standby mode and changes the mode switching signal to the high level. The comparator 1 responsively becomes operable and the multiplexer 3 selects the output signal of the terminal C.
When the comparators 1 and 2 change its state between the operable state and the inoperable state, the output signal levels of the comparators 1 and 2 become unstable for a short period of time as shown by hatching FIG. 7. This period of instability of the comparator 2, which is a low power consumption type, tends to become longer than that of the comparator 1. As a result, the microcomputer 4 may receive inappropriate signal in this period and operate erroneously.